1. Field of the Invention
The present invention relates to a method of evaluating high fatigue strength in high tensile strength steel, and creation of a high fatigue strength steel produced in accordance with the aforementioned evaluation method. More specifically, the present invention relates to a method of evaluating high fatigue strength in high tensile strength steel and creation of a high fatigue strength steel, which are useful for material designing and production of high tensile strength steel having high strength of giga-cycle fatigue. The high tensile strength steel having high giga-cycle fatigue strength is advantegously used for reducing weight and size of vehicle parts and others.
2. Description of the Related Art
High tensile strength steel having excellently high strength of giga-cycle fatigue has been on demand, in order to reduce weight and size of vehicle parts and others. However, in the case of high tensile strength steel of 1200-MPa-plus grade, fatigue strength saturates or rather decreases at the giga-cycle range, since internal fracture; that is, a fish eye-type fracture initiates at inclusions or internal facets. Therefore, it is not technologically easy to produce high tensile strength steel having high fatigue strength. Further, a method of designing high tensile strength steel having higher fatigue strength is hardly known, because a mechanism of the internal fracture is quite complicated.
An example of a method of designing high tensile strength steel includes the steps of: measuring size (area) and/or properties of flaws of the inclusion and internal facet; and calculating a fatigue limit from a correlation equation which defines a relationship between the flaw area and the fatigue limit.
However, as this method calculates only an approximate fatigue limit value by simplifying the complicated mechanism of the internal fracture, the method is by no means a designing method for high tensile strength steel having a higher fatigue strength.
A new attempt has been made in recent years, as one method of solving the complicated mechanism of the internal fracture, in which an optically dark area (ODA), which is presumably the fracture area developed by hydrogen around the inclusion, prior austenite grain boundary, dislocation and others, is observed and the relationship between the ODA (flaw area) and fatigue strength is analyzed. If the formation mechanism of ODA (flaw area) is made clear in the future, this discovery may contribute to providing a method of designing high tensile strength steel having excellent fatigue strength. However, this discovery has not yet been fully made and there does not exist a clear prospect of specifically designing such a method.
The invention of the present application has been achieved in consideration of the aforementioned problems of the prior art. The present invention has as an object providing a method of evaluating high fatigue strength in high tensile strength steel, and a high fatigue strength steel itself, in which method the relationship between the flaw size (area) of ODA and the fatigue strength is considered.
In order to solve the aforementioned problems, the present invention proposes, as a first aspect, a method of evaluating a high fatigue strength in high tensile strength steel, comprising the steps of obtaining values of tensile strength "sgr"B (the unit thereof is MPa) and Vickers hardness Hv of the steel; and estimating, in designing the high fatigue strength steel, that the fatigue strength "sgr"w (the unit thereof is MPa) of the steel satisfies either "sgr"wxe2x89xa70.5 "sgr"B or "sgr"wxe2x89xa71.6 Hv, when a fracture origin is located only at a surface of the steel and a square root of the flaw area, (area)xc2xd (the unit thereof is m), contained in the steel is no larger than 45.8/"sgr"B2 or 4.47/Hv2.
The present invention proposes, as a second aspect, a method of evaluating a high fatigue strength in high tensile strength steel, comprising the steps of: obtaining values of tensile strength "sgr"B (the unit thereof is MPa) and Vickers hardness Hv of the steel; measuring flaw area of an inclusion, when a fracture origin is located inside the steel; and estimating, in designing high fatigue strength, that the fatigue limit "sgr"w (the unit thereof is MPa) of the steel satisfies a condition that "sgr"wxe2x89xa73.38 (areai)xe2x88x92xc2xc.
The present invention propose, as a third aspect, a method of evaluating a high tensile strength structure, which method enables evaluation of high fatigue strength according to the aforementioned first or second aspects, which method comprises the steps of: measuring a maximum inhomogeneous structure area, (areamax,m), when the structure has been made homogeneous (i.e., the size of the inhomogeneous structure has been made small) or the structure has been made minute (i.e., a block width thereof has been reduced); setting a distribution of the maximum-minimum range of the maximum inhomogeneous structure area, (areamax,m)xc2xd, (the unit thereof is xcexcm) within the range defined by the line: (areamax,m)xc2xd=0 and the line: (areamax,m)xc2xd=0.9403y+4.571 (y is a standardizing parameter, and a test standard area SO=6.2xc3x9710xe2x88x929 m2); and setting a distribution of the maximum-minimum range of the maximum block width dmax (the unit thereof is xcexcm) within the range defined by the line: dmax=0 and the line: dmax=0.217y+0.701 (y is a standardizing parameter, and a test standard area is 1xc3x9710xe2x88x9210 m2).
The present invention proposes, as a fourth aspect, a method of producing a high fatigue strength steel, which method comprises producing steel according to the method of evaluating a high fatigue strength in high tensile strength steel of the aforementioned first and second aspects, and the method of evaluating a high tensile strength structure of the aforementioned third aspect.
The present invention proposes, as a fifth aspect, a method of producing a high fatigue strength, which method comprises producing steel according to the method of the aforementioned first aspect, by subjecting steel to a heat treatment for tempering the steel in a high vacuum of at least 2xc3x9710xe2x88x926 Pa.
Further, the present invention proposes, as a sixth aspect, a high fatigue strength material produced according to the method of evaluating a high fatigue strength in high tensile strength steel of the aforementioned first and second aspects, and the method of evaluating a high tensile strength structure of the aforementioned third aspect.